THE ABSTRACTIONS, SPECIES AND GENUS. 249 



distinguish these seventeen-gilled crayfishes, as a whole, 

 from the eighteen-gilled species ; and this is effected by 

 changing the generic name. They are no longer called 

 Astacus, but Cambarus (fig. 63). 



All the individual crayfish referred to thus far, there- 

 fore, have been sorted out, first into the groups termed 

 species ; and then these species have been further sorted 

 into two divisions, termed genera. Each genus is an 

 abstraction, formed by summing u p th e common char- 

 acters of the species which it includes, just as each 

 species is an abstraction, composed of the common 

 characters of the individuals which belong to it ; and 

 the one has no more existence in nature than the other. 

 The definition of the genus is simply a statement of 

 the pjun of structure which is .common to all the species 

 included under that genus ; just as the definition of the 

 species is a statement of the common plan of structure 

 which runs throughout the individuals which compose 

 the species. 



Again, crayfishes are found in the fresh waters of the 

 Southern hemisphere ; and almost the whole of what 

 has been said respecting the structure of the English cray- 

 fish applies to these ; in other words, their general plan is 

 the same. But, in these southern crayfishes, the podo- 

 branchise have no distinct lamina, and the first somite of 

 the abdomen is devoid of appendages in both sexes. The 

 southern crayfishes, like those of the Northern hemi- 

 sphere, are divisible into many species ; and these species 

 12 



